This invention relates to an apparatus for manufacturing containers, lids and other plastic items by thermoforming a plastic sheet material on a continuous run basis. It relates in particular to small, relatively high-volume machines which operate using molds of single or few cavities.
In general, in thermoforming processes, a sheet of thermoplastic material is carried across one-half of a mold. The thermoplastic sheet is heated, and at a specific time, the other half of the mold is inserted into the first half at considerable pressure to form the desired shape in the thermoplastic sheet. This process may be assisted by a vacuum on one side and pressure on the other to maintain the configuration of the mold and by cooling. It has been known to use large molds, which are usually multi-cavity for molds of small parts. Smaller molds are generally single-cavity or have relatively few cavities. In general, thermoforming machines have been increasing in size in an attempt to lower unit manufacturing costs. While this is satisfactory for an extremely high-volume operation, it is not satisfactory for a lower volume operation or thermoforming job.
The basic difficulty with building a small thermoforming machine using smaller molds is that the unit consumption of thermoplastic material increases as the mold is decreased in size. This is because most thermoforming machines carry the thermoplastic material through the machine and across the mold by a chainpin system which grips the thermoplastic material on each side of the sheet. A chainpin generally punctures the material one-quarter to one-half inch in from the edge, consuming three-quarters to one inch on each side of the sheet, which is trim scrap and is waste in the manufacturing process. Given a total sheet width of thirty inches on a standard thermoplastic material roll, one and one-half-inch total waste is approximately five percent of the material. However, with a small mold and a ten-inch sheet width, the same amount of waste increases to fifteen percent of the total. In addition, since present thermoforming machines run the sheet of thermoplastic material through the mold horizontally, the material must be held on the edges with the chainpin. Since the material is heated, as noted above, it becomes soft and pliable after passing through the radiant heaters and thus begins to sag as it is carried into the forming station. Sagging creates problems known in the trade as "bridging", "webbing", "thinning" and "dragging" which result in thin walls in various spots in the molded item, premature cooling of other spots and lower overall quality of the finished product.
Another present process exists which does not use chainpins as material carriers. This is a contact heat thermoformer wherein the material is heated in the actual forming station, where it is held while being heated on a flat-bottom plate. This process works well with specific plastics which are heavily oriented and shrink when they are heated. The process has the distinct limitations of being only feasible for running on female molds, and it cannot be plug-assisted due to the required flat heating plate. Speeds are also limited somewhat in that the material must be heated when it is in the forming station, although initial pre-heating can be done. These machines are generally used for very low profile containers such as thin-gauge, high-volume food trays and pill and tablet packs for pharmaceuticals. The greatest limitation in this method is the lack of plug-assisting, which limits these machines to shallow and very simple parts. As noted above, the contact thermoforming machine is used extensively for forming oriented polystyrene (OPS) which has a tremendous orientation, i.e., it tends to shrink when heated.
A final apparatus which is presently in use is called the cap-press machine. These machines, which are constructed in Holland, are specifically used to thermoform caps for round containers, using 0.01 to 0.015 polyvinylchloride (PVC). Such machines feed the material vertically from top to bottom, eliminating the need for the chainpin. These machines, however, are cam-driven, which minimizes their adjustability and versatility. In addition, due to the fixed cam cycle, it is not possible to obtain a plug-assist on such machines. This machine also requires use of a "cut-in-place" method, producing an accurately cut part but requiring very expensive tooling. The machine uses either contact heat or radiant heat in a pre-heat mold.
The object, therefore, of the present invention is to eliminate extensive material waste in connection with thermoforming involving smaller machines with single or multiple cavity mold for use with thermoplastic material sheets. It is also an object of this invention to move the thermoplastic sheet through the forming station in a manner so as to retard sagging and other problems such as bridging, webbing, thinning and dragging over the mold. A further object of this invention is to produce an apparatus having the ability to be plug-assisted to produce a variety of thermoformed objects. Another object of the invention is to produce a thermoforming apparatus capable of obtaining a relatively high speed for the size of the machine with greatly improved control of the process. A further object is to produce a thermoforming machine which can be used effectively with small and relatively inexpensive tooling, and which is adjustable to fit various sizes of tooling and different sizes of thermoplastic sheet materials. A further object of the invention is to be able to clamp closely around each mold cavity to control quality and minimize the material consumed. Another object of the invention is to produce an apparatus having a clamping mechanism which can remain in position while the sheet is indexed past it to allow for a relatively inexpensive clamp mechanism that is merely mounted to the machine and has no moveable parts to it.